1. Field of the Invention
The present invention relates to marine sonar doppler navigation and guidance apparatus and more particularly to acoustic radiators and receivers for such apparatus.
2. Description of the Prior Art
Doppler sonar equipments used for speed and distance measurement aboard ships are well known in the art. Electro-acoustic transducers are used to generate and receive sound waves in the water. The acoustic energy is beamed into the water at some angle other than 90.degree. with respect to the direction of the vessel's forward velocity so that a component of the velocity coincides with the beam direction. An example of such beam orientation, employed in a "Janus system", wherein two acoustic beams are used for added accuracy, is illustrated in U.S. Pat. No. 3,795,893 entitled "Doppler Speed Log", issued to Jacob A. Kritz and Seymour D. Lerner, and assigned to the present assignee.
Under many conditions of ship operation, air bubbles are entrained under the hull and swept aft. The bubbles adjacent the hull are in a boundary layer of the stream moving past the ship. The velocity of this boundary layer is appreciably less than the free stream velocity remote from the ship's hull, which is the velocity to be measured. The air bubbles represent excellent reflectors of sound energy. In prior art transducers, energy is radiated and received not only along the desired principal lobe, but also along various minor side lobes disposed close to the ship's hull. Energy in these side lobes is often reflected from the air bubbles which are close to the hull surface and present a spurious reflected signal which is comparable to that received from the intended region in the principal lobe. These spurious signals not only cause inaccurate operation, but also result in a high degree of instability in the instrument reading because of the continually varying bubble density.